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Combustion of Carbon
Owing to their importance as fuel, carbonaceous materials have for centuries been the subject of scientific consideration. For some time prior to the discovery of oxygen, carbon or charcoal was regarded as composed mainly of the essence of combustibility, and Stahl (c. 1697) considered it to be almost pure phlogiston. On this theory, the fact that only a certain quantity of charcoal could burn in a limited supply of air was readily explained on the assumption that phlogiston could not leave a substance unless it had somewhere to go. The air could only absorb a definite amount, and when once fully phlogisticated behaved like a saturated body and refused to take up any more.
The discovery of oxygen by Priestley and independently by Scheele in the second half of the eighteenth century enabled Lavoisier to offer an entirely new explanation for the phenomena. The carbon was assumed to combine direct with oxygen to form the dioxide
(i) C + O2 = CO2,
and the fact that carbon monoxide was found to result in the presence of excess charcoal, was accounted for by reduction of the carbon dioxide
(ii) CO2 + C = 2CO.
For more than half a century this theory was accepted almost without question, but in 1872 Sir Lowthian Bell concluded that the theory was inadequate in so far as the Combustion of Carbon or coke in a blast furnace was concerned. He suggested that "carbon monoxide and not carbon dioxide is the chief, if not the exclusive and immediate, action of the hot blast on the fuel." If this view is accepted, carbon dioxide is to be regarded as an oxidation product of the monoxide rather than of carbon itself, and carbon monoxide as the primary oxidation product of carbon instead of a reduction product of the dioxide. Thus:
(i) 2C + O2 = 2CO;
(ii) 2CO + O2 = 2CO2.
Bell's theory received unexpected support from the work of C. J. Baker some fifteen years later. This investigator studied the effect of admission of oxygen to charcoal that had previously been thoroughly exhausted of air and moisture by heating to redness in an evacuated tube containing phosphorus pentoxide. On admitting dry oxygen to the system, adsorption took place, and a temperature of 450° C. was required to expel it. It then escaped mainly as carbon monoxide. In the presence of moisture carbon dioxide was formed, but the more thoroughly free from moisture the substances were, the less the amount of carbon
dioxide produced. This, coupled with the fact, proved by Baker, that dried carbon dioxide is reduced by dry carbon only with difficulty to the monoxide, strongly supports the conclusion that under these conditions carbon burns directly to the monoxide.
Still further support was forthcoming the following year from the researches of H. B. Baker, who found that no visible Combustion of Carbon occurred when thoroughly dry oxygen was led over highly purified sugar charcoal at bright-red heat. That combination had taken place, however, was clear from the resulting gaseous mixture, namely:
Johnson and MTntosh found from 6.2 to 8.9 per cent, of carbon monoxide in the gases evolved during the combustion of a mixture of carbon with excess of potassium chlorate, both in air and in a vacuum. As the temperature was only of the order of 1000° C., the authors argue that the monoxide could not have resulted from the thermal decomposition of the dioxide, so that the monoxide would appear to be the first product in the combustion of carbon.
In 1896 Dixon showed that the rate of explosion of cyanogen in oxygen reaches a maximum when the gases are in molecular proportions. Thus:
C2N2 + O2 = N2 + 2CO.
Further, the pressure developed during the explosion is greater, and the reaction proceeds more rapidly, than when sufficient oxygen is present to convert the carbon into the dioxide.
It would appear, therefore, that in so far as gaseous carbon is concerned, carbon monoxide is the initial product.
For many years chemists appear to have been satisfied with one or other of these theories. In 1912 attention was again directed to the subject by Rhead and Wheeler, who point out that if it could be shown that either the reaction
C + O2 = CO2
or
CO2 + C = 2CO
proceeds at a temperature at which either
2C + O2 = 2CO
or
2CO + O2 = 2CO2
takes place with inappreciable velocity, a decision between the two foregoing theories could be arrived at. Although unable to arrive at a complete solution of the problem in this simple manner, Rhead and Wheeler succeeded in showing that -
The conclusion appears inevitable that both the monoxide and the dioxide are produced simultaneously. In other words, neither gas is the primary product of oxidation in that it takes precedence over the other. The two previous theories are therefore to be regarded as correct, each as far as it goes. The question now arises as to the nature of the reaction between carbon and oxygen, and this was discussed by Rhead and Wheeler the following year. It was found that charcoal that had been heated to 950° C. in an evacuated vessel and allowed to cool, readily adsorbs oxygen at all lower temperatures, the rate of adsorption being very rapid during the first few seconds, after which the velocity gradually slows down, but continues for several hours (see fig.). The total amount of occluded oxygen increases with fall of temperature, and is approximately constant for any given temperature for the particular specimen of charcoal under observation. This oxygen cannot be removed by exhaustion alone, but only by increasing the temperature of the carbon during exhaustion. When quickly released in this manner it appears, not as oxygen, but as carbon dioxide and carbon monoxide. The proportions in which it appears in these two oxides when completely removed depend on the temperature at which the carbon has been heated during oxygen-fixation.
If, for example, the temperature is raised, say from 300° to 350° C., oxygen is evolved in vacuo, corresponding to the amount AB in fig., as a mixture of monoxide and dioxide until the saturation limit of the charcoal at this higher temperature has been reached, and then ceases.
The authors suggest that this is more than a purely physical " fixation " of oxygen, being in all probability the outcome of a physico-chemical attraction between oxygen and carbon. Physical, inasmuch as it seems hardly possible to assign any definite molecular formula to the complex formed, which, indeed, shows progressive variation in composition; chemical, in that no isolation of the complex can be effected by physical means.
It would appear, therefore, that the first product of combustion of carbon is a loosely formed physico-chemical complex, which can be regarded as an unstable compound of carbon and oxygen of an at present unknown composition, CxOy. It is probable that no definite formula can be assigned to this complex.
Stated mainly in the words of the authors themselves, the conception of what takes place during the combustion of carbon is, briefly, as follows: each oxygen molecule that comes into collision with the carbon becomes "fixed," in so far as it is rendered incapable of further progress by the attraction of several carbon molecules. There is as yet no absolute knowledge of the number of atoms contained in the carbon molecule. The formation of benzenehexacarboxylic acid (mellitic acid) by the oxidation of either amorphous carbon or graphite, warrants the assumption that the carbon molecule contains not fewer than twelve atoms, and may be even more complex. It is conceivable, therefore, the authors point out, that in the oxidation of carbon the oxygen molecule actually enters the carbon molecule, a rearrangement of atoms taking place. However, for the present it is sufficient to assume that several carbon molecules hold one oxygen molecule, in bond as it were, and do not allow it to escape in conjunction with one of their atoms. A considerable evolution of heat takes place during this attachment of oxygen molecules, so much so that some of them eventually acquire sufficient energy to seize hold of a carbon atom and depart with it as carbon dioxide. Some of them become torn apart in the process - become atomised - and leave the carbon molecule as carbon monoxide.
This formation of a complex, and partial decomposition as fresh oxygen molecules become attached, goes on until the carbon becomes " saturated," the products of combustion during this period (a comparatively short one) being CxOy, CO2, and CO. After the carbon has become saturated there is an alternate formation and decomposition of the complex. Each oxygen molecule that impinges on the carbon is at once seized hold of to form the complex, but the energy set free when this occurs decomposes an equivalent proportion of the complex formed from previous oxygen molecules. So that, finally, when air is passed over saturated carbon maintained at a constant temperature by the application of an external source of heat, carbon dioxide and carbon monoxide appear in the products of combustion in volume sufficient to account for the total volume of oxygen in the air originally passed.
In the normal burning of carbon, therefore, the carbon dioxide and carbon monoxide found as the apparently primary products of combustion, arise from the decomposition, at the temperature of combustion, of a complex the formation of which is the first result of the encounters between oxygen and carbon molecules.
The idea of " oxygenation " of the combustible as a preliminary to definite chemical reaction is not without precedent.
Indeed, it receives strong support from the work of Bone on the combustion of hydrocarbon gases. Although not definitely proved, this attractive theory is certainly a most suggestive one; it not only fits in well with known facts, but is in harmony with the various points urged in favour of each of the two earlier theories.
The discovery of oxygen by Priestley and independently by Scheele in the second half of the eighteenth century enabled Lavoisier to offer an entirely new explanation for the phenomena. The carbon was assumed to combine direct with oxygen to form the dioxide
(i) C + O2 = CO2,
and the fact that carbon monoxide was found to result in the presence of excess charcoal, was accounted for by reduction of the carbon dioxide
(ii) CO2 + C = 2CO.
For more than half a century this theory was accepted almost without question, but in 1872 Sir Lowthian Bell concluded that the theory was inadequate in so far as the Combustion of Carbon or coke in a blast furnace was concerned. He suggested that "carbon monoxide and not carbon dioxide is the chief, if not the exclusive and immediate, action of the hot blast on the fuel." If this view is accepted, carbon dioxide is to be regarded as an oxidation product of the monoxide rather than of carbon itself, and carbon monoxide as the primary oxidation product of carbon instead of a reduction product of the dioxide. Thus:
(i) 2C + O2 = 2CO;
(ii) 2CO + O2 = 2CO2.
Bell's theory received unexpected support from the work of C. J. Baker some fifteen years later. This investigator studied the effect of admission of oxygen to charcoal that had previously been thoroughly exhausted of air and moisture by heating to redness in an evacuated tube containing phosphorus pentoxide. On admitting dry oxygen to the system, adsorption took place, and a temperature of 450° C. was required to expel it. It then escaped mainly as carbon monoxide. In the presence of moisture carbon dioxide was formed, but the more thoroughly free from moisture the substances were, the less the amount of carbon
dioxide produced. This, coupled with the fact, proved by Baker, that dried carbon dioxide is reduced by dry carbon only with difficulty to the monoxide, strongly supports the conclusion that under these conditions carbon burns directly to the monoxide.
Still further support was forthcoming the following year from the researches of H. B. Baker, who found that no visible Combustion of Carbon occurred when thoroughly dry oxygen was led over highly purified sugar charcoal at bright-red heat. That combination had taken place, however, was clear from the resulting gaseous mixture, namely:
| Oxygen | 58.1 % |
| Carbon monoxide | 39.5 % |
| Carbon dioxide | 2.2 % |
Johnson and MTntosh found from 6.2 to 8.9 per cent, of carbon monoxide in the gases evolved during the combustion of a mixture of carbon with excess of potassium chlorate, both in air and in a vacuum. As the temperature was only of the order of 1000° C., the authors argue that the monoxide could not have resulted from the thermal decomposition of the dioxide, so that the monoxide would appear to be the first product in the combustion of carbon.
In 1896 Dixon showed that the rate of explosion of cyanogen in oxygen reaches a maximum when the gases are in molecular proportions. Thus:
C2N2 + O2 = N2 + 2CO.
Further, the pressure developed during the explosion is greater, and the reaction proceeds more rapidly, than when sufficient oxygen is present to convert the carbon into the dioxide.
It would appear, therefore, that in so far as gaseous carbon is concerned, carbon monoxide is the initial product.
For many years chemists appear to have been satisfied with one or other of these theories. In 1912 attention was again directed to the subject by Rhead and Wheeler, who point out that if it could be shown that either the reaction
C + O2 = CO2
or
CO2 + C = 2CO
proceeds at a temperature at which either
2C + O2 = 2CO
or
2CO + O2 = 2CO2
takes place with inappreciable velocity, a decision between the two foregoing theories could be arrived at. Although unable to arrive at a complete solution of the problem in this simple manner, Rhead and Wheeler succeeded in showing that -
- Some carbon monoxide is produced during the oxidation of carbon at low temperatures, under conditions that do not admit of the reduction of carbon dioxide by carbon.
- Carbon dioxide is produced at low temperatures in quantities that cannot be entirely accounted for on the assumption that the monoxide is first formed and subsequently oxidised.
 |
| Oxygen absorption by exhausted charcoal |
If, for example, the temperature is raised, say from 300° to 350° C., oxygen is evolved in vacuo, corresponding to the amount AB in fig., as a mixture of monoxide and dioxide until the saturation limit of the charcoal at this higher temperature has been reached, and then ceases.
The authors suggest that this is more than a purely physical " fixation " of oxygen, being in all probability the outcome of a physico-chemical attraction between oxygen and carbon. Physical, inasmuch as it seems hardly possible to assign any definite molecular formula to the complex formed, which, indeed, shows progressive variation in composition; chemical, in that no isolation of the complex can be effected by physical means.
It would appear, therefore, that the first product of combustion of carbon is a loosely formed physico-chemical complex, which can be regarded as an unstable compound of carbon and oxygen of an at present unknown composition, CxOy. It is probable that no definite formula can be assigned to this complex.
Stated mainly in the words of the authors themselves, the conception of what takes place during the combustion of carbon is, briefly, as follows: each oxygen molecule that comes into collision with the carbon becomes "fixed," in so far as it is rendered incapable of further progress by the attraction of several carbon molecules. There is as yet no absolute knowledge of the number of atoms contained in the carbon molecule. The formation of benzenehexacarboxylic acid (mellitic acid) by the oxidation of either amorphous carbon or graphite, warrants the assumption that the carbon molecule contains not fewer than twelve atoms, and may be even more complex. It is conceivable, therefore, the authors point out, that in the oxidation of carbon the oxygen molecule actually enters the carbon molecule, a rearrangement of atoms taking place. However, for the present it is sufficient to assume that several carbon molecules hold one oxygen molecule, in bond as it were, and do not allow it to escape in conjunction with one of their atoms. A considerable evolution of heat takes place during this attachment of oxygen molecules, so much so that some of them eventually acquire sufficient energy to seize hold of a carbon atom and depart with it as carbon dioxide. Some of them become torn apart in the process - become atomised - and leave the carbon molecule as carbon monoxide.
This formation of a complex, and partial decomposition as fresh oxygen molecules become attached, goes on until the carbon becomes " saturated," the products of combustion during this period (a comparatively short one) being CxOy, CO2, and CO. After the carbon has become saturated there is an alternate formation and decomposition of the complex. Each oxygen molecule that impinges on the carbon is at once seized hold of to form the complex, but the energy set free when this occurs decomposes an equivalent proportion of the complex formed from previous oxygen molecules. So that, finally, when air is passed over saturated carbon maintained at a constant temperature by the application of an external source of heat, carbon dioxide and carbon monoxide appear in the products of combustion in volume sufficient to account for the total volume of oxygen in the air originally passed.
In the normal burning of carbon, therefore, the carbon dioxide and carbon monoxide found as the apparently primary products of combustion, arise from the decomposition, at the temperature of combustion, of a complex the formation of which is the first result of the encounters between oxygen and carbon molecules.
The idea of " oxygenation " of the combustible as a preliminary to definite chemical reaction is not without precedent.
Indeed, it receives strong support from the work of Bone on the combustion of hydrocarbon gases. Although not definitely proved, this attractive theory is certainly a most suggestive one; it not only fits in well with known facts, but is in harmony with the various points urged in favour of each of the two earlier theories.
Composition of the Complex
An attempt was made to determine the quantity of oxygen adsorbed by a sample of charcoal at 300° C. The result indicated an adsorption of 0.16 gram of oxygen by 12 grams of carbon, corresponding to a formula of C100O. This, of course, only refers to the temperature chosen, namely, 300° C. The relative proportions of carbon dioxide and monoxide evolved on raising the temperature of saturated charcoal were found to vary with the initial temperature, so that it appears impossible to determine the values for x and y from the available data.
As mentioned above, however, Baker has shown that carbon dioxide, when thoroughly dried by prolonged contact with phosphorus pentoxide, is not reduced by carbon even at bright-red heat. On the other hand, Rhcad and Wheeler have shown that the complex CxOy is readily formed from its dry constituents. These two observations suggest a method by means of which the ratio x/y might be discovered, if not exactly at any rate approximately. For by exposing thoroughly dry charcoal which has been exhausted at, say, 1100° C. to thoroughly dry oxygen at a lower temperature, the complex CxOy, should be obtained in an equally dry condition. On raising the temperature and pumping off the two oxides of carbon, these should be obtained in the proportions in which the atoms of oxygen and carbon are distributed in the complex, inasmuch as none of the carbon dioxide once produced in this way can be reduced to monoxide during the experiment, as undoubtedly occurs in the case of the moist gases.
Only two series of experiments have been carried out with this object in view, the results being inconclusive.
As mentioned above, however, Baker has shown that carbon dioxide, when thoroughly dried by prolonged contact with phosphorus pentoxide, is not reduced by carbon even at bright-red heat. On the other hand, Rhcad and Wheeler have shown that the complex CxOy is readily formed from its dry constituents. These two observations suggest a method by means of which the ratio x/y might be discovered, if not exactly at any rate approximately. For by exposing thoroughly dry charcoal which has been exhausted at, say, 1100° C. to thoroughly dry oxygen at a lower temperature, the complex CxOy, should be obtained in an equally dry condition. On raising the temperature and pumping off the two oxides of carbon, these should be obtained in the proportions in which the atoms of oxygen and carbon are distributed in the complex, inasmuch as none of the carbon dioxide once produced in this way can be reduced to monoxide during the experiment, as undoubtedly occurs in the case of the moist gases.
Only two series of experiments have been carried out with this object in view, the results being inconclusive.
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